Bandgap reference circuits are commonly used to generate a stable reference voltage from the silicon bandgap. Bandgap reference generator circuits may be used in DC-DC converters, Analog-to-Digital Converters (ADC), low dropout drivers, and many other kinds of analog circuits.
The base-to-emitter voltage Vbe in a PNP transistor, shown in equation EQN1,
                                          V            be                    =                                    V              T                        ⁢            ln            ⁢                                          I                c                                            A                *                                  J                  s                                                                    ,                            EQN        ⁢                                  ⁢        1            
where VT is thermal voltage, A is the emitter-base junction area, and Js is the current density. The base-emitter voltage Vbe is relatively constant because a large collector current Ic variation only causes a small Vbe variation. A pair of ratioed PNP transistors can be used to sink current in a voltage divider network that generates the reference voltage. A feedback loop can be included with an op amp that has compare inputs tapped from nodes within the voltage divider network. Many variations of this basic circuit are in use.
The basic bandgap reference circuit creates a reference voltage that is independent of temperature, supply voltage, and process variations. However, the feedback loop can introduce an offset that does vary with the process. These process variations can be compensated for by trimming the resistance value of a resistor in the voltage divider network.
After the circuit is fabricated, a test probe is dropped onto a pad on the voltage reference node or another related node. The reference voltage is measured using the test probe. The resistance value is trimmed or adjusted by blowing fuses or trimming resistors with a laser, programming registers that control the resistance value, or by some other method. The reference voltage is measured again, and the resistance value again adjusted. Several iterations may be used to fine-tune the reference voltage by successively trimming smaller resistance values.
While trimming is useful, it is difficult to precisely tune the resistance value. The reference voltage may be overshot without any way to compensate when permanent fuses are blown. Trimming is often one-dimensional, either increasing or decreasing the reference voltage.
What is desired is a bi-directional trimming circuit for a bandgap reference circuit. A reference circuit that can trim the reference voltage both up and down is desired.